docs: plan both telemetry changes down to the task level

Twelve tasks across the two plans, each with the files it touches, the
evidence it has to produce, and the command that proves it. #13 goes
first: both branches edit config.py and client.py, and #12's
partitioning template leans on the schema SQL helper and the untargeted
conflict clause that #13 introduces.

Writing the cap plan surfaced a trap worth its own guard. digest_messages
appends the very same dict when a message's content is not a list, so
the telemetry copy, the caller's messages and the cache key all share
one object -- capping in place would poison the caller's request and the
cache key at once, silently. Two red-line tests now pin that down, and
the plan asks for an in-place version to be written and run first, to
prove the tests actually catch it.

research-wiki/plans/plan-issue12-telemetry-retention.md

@@ -0,0 +1,17 @@
+---
+type: plan
+node_id: plan:plan-issue12-telemetry-retention
+title: "实现计划: issue12-telemetry-retention"
+date: 2026-08-19
+---
+
+# 实现计划: issue12-telemetry-retention
+
+
+正文: `2026-08-19-issue12-telemetry-retention.md`。实现 [[design:issue12-telemetry-retention]]。
+
+五个任务: ① 截断函数 + emitter `text_cap` 必填 + 三构造点; ② 配置与装配; ③ `tools/telemetry_retention.py`(默认 dry-run); ④ README 生产部署模板 + 其真实 PG 机械化验收; ⑤ CHANGELOG 与 wiki。
+
+**前置**: issue #13 须先合并(两条分支都改 `config.py`/`client.py`,且分区模板依赖 #13 的 `telemetry_schema_sql()` 与无冲突目标写入)。
+
+写计划时挖出的实现陷阱: `digest_messages` 对 content 非 list 的消息**原样 append 同一个 dict**,遥测拿到的与调用方传入的、缓存 key 用的是同一份对象——`_cap_messages` 若就地改,会同时污染调用方 messages、后续重试请求体与缓存写入 key,且全程无报错。计划已为此设两条红线用例,并要求先写一版就地改的实现证明红线能抓住它。